1. Field of the Invention
The invention relates to an apparatus for combatting intersymbol interference and noise introduced into a data signal transmitted at a symbol rate 1/T by a transmission channel having a memory span LT corresponding to a number of L consecutive data symbols, which apparatus comprises:
a receive filter for forming a delayed version of the transmitted data signal and for suppressing noise therein; a first decision circuit for forming preliminary symbol decisions in response to the transmitted data signal;
a second decision circuit for forming the final symbol decisions;
compensating means comprising a feedforward section for forming a first compensating signal for pre-cursive intersymbol interference in response to a number of consecutive preliminary symbol decisions, and a feedback section for forming a second compensating signal for post-cursive intersymbol interference in response to a number of consecutive final symbol decisions; and
a combining circuit for forming an input signal for the second decision circuit for the delayed and filtered version of the transmitted data signal and the sum of the first and second compensating signals.
2. Related Art
Such an arrangement is known from an article entitled "Adaptive Cancellation of Nonlinear Intersymbol Interference for Voiceband Data Transmission" by E. Biglieri et al., published in IEEE Journal on Selected Areas in Communications, Vol. SAC-2, No. 5, Sept. 1984, pp. 765-777, in particular FIG. 7 and the associated description. This article describes several implementations of arrangements for combatting both linear and non-linear intersymbol interference as well as noise in data signals.
The implementation shown in FIG. 7 of this article relates to a non-linear canceller in which noise is suppressed by using a receive filter in the form of a so-called Matched Filter, whose impulse response is the time-reverse of that of the transmission channel, and in which it is attempted to cancel intersymbol interference by synthesizing a copy of it and substracting this copy from the output signal of the matched filter. From this article it appears that the memory span of the feedforward section as well as the feedback section has to be equal to that of the transmission channel to achieve the optimum transmission quality.
This conclusion also follows from an article entitled "A Unified Theory of Data-Aided Equalization" by M. S. Mueller and J. Salz, published in Bell System Technical Journal, Vol. 60, No. 9, Nov. 1981, pp. 2023-2038, in which theoretical considerations are devoted to techniques for simultaneously combatting intersymbol interference and noise. In this article, the occurring intersymbol interference is assumed to be linear and the symbol decisions applied to the compensating means are assumed to be without errors, and the arrangement is optimized according to a criterion for minimizing the mean-square error. Mueller and Salz likewise conclude that a degradation of the transmission quality is bound to occur if the memory span of the feedforward section or feedback section is selected to be smaller than that of the transmission channel. However, in practice, the symbol decisions applied to the compensating means are not without errors. The effects of these erroneous symbol decisions on the final symbol decisions, a phenomenon denoted as error propagation, leads to a degradation of the transmission quality. This clearly appears from an article entitled "On the Performance and Convergence of the Adaptive Canceller of Intersymbol Interference in Data Transmission" by K. Wesolowski, published in IEEE Transactions on Communications, Vol. COM-33, No. 5, May 1985, pp. 425-432, which reports on measurements of error probabilities in the final symbol decisions for the arrangement considered here, which measurements were obtained for different transmission channels, the respective memory spans of the compensating means each time being selected so as to be equal to that of the transmission channel.
Despite the fact that in the articles by both Wesolowski and Biglieri et al, the detrimental effects of error propagation are recognized, neither of these articles provides a solution to this problem.